Thanks, that worked. I asked over at infopop and they suggested this:
Addpost.php
// -----------------------------
// Always markup the signature
$printsig = $user['U_Signature'];
$printsig = $html -> do_markup($printsig);
// -----------------------
// Add the sig to the post
$PrintBody = "$PrintBody<br><br>$printsig";
Which I would then change to look like this:
// -----------------------------
// Always markup the signature
$printsig = $user['U_Signature'];
$printsig = $html -> do_markup($printsig);
// -----------------------
// Add the sig to the post
$PrintBody = "$PrintBody<br>---------------------------------<br>$printsig";
But it didn't work, but I like the use of the horizontal rule rather than the dashes anyway. Thanks!